1. Field of the Invention
The present invention relates to the visual, non-contact edge inspection of semiconductor wafers, e.g. silicon wafers, at different stages of wafer processing such as slicing, lapping, edge-shaping (grinding), etching, and polishing which is generally performed by trained operators. The inspection occurs under a bright light, from halogen or fluorescent lamps, with the naked eye, or by using a magnifier lens (2.times. to 4.times. magnification).
2. The Prior Art
In this type of inspection, the submilimetric size of some edge defects makes inefficient the visual inspection with the naked eye.
A better alternative to such methods would be to perform the inspection with a video camera microscope with a special illumination light source. This light source can be a gooseneck fiber or a fiber optic ring attached parallel and coaxial to the camera objective.
Most wafer edge inspection equipment currently available uses video cameras with the microscope objective attached. Also there are visible, fluorescent or halogen lamps (Hologenix), or red laser beams (Champman), which are used as light sources for detecting/monitoring the defects and characterizing the edge texture roughness.
For these instruments, the light beam impinges upon the wafer edge surface at one particular angle, 90 degrees or less. The reflected light that contains the wafer edge texture information is then captured by the video camera.
Generally speaking, the wafer edge illumination methods fall in two categories: 1) dark-field illumination (DF) and 2) bright-field illumination (BF). A bright-field illumination system examines directly reflected light (close to the normal incidence beam), while a dark-field illumination system examines light scattered off the wafer (under small angle incidence beam), almost parallel with the edge.
Bright-field illumination means illuminating the edge at normal incidence angle, and looking at the resulting reflected image. This configuration can be described as a "co-axial" or "through-the-lens" geometry because the light source has a solid angle with respect to the part being observed. For example, a 25 mm light source (laser or light bulb), at a distance of 100 mm from the part being imaged, has a solid angle approximately equal to arctan 25/100=14 degrees.
For flat specular surfaces, such as polished flat edges, the minimum dimension for a light source WBFmin, held at a distance D=(D1-D2) of the camera lens, should be twice the field-of-view width V, plus the camera lens entrance pupil diameter (aperture) A (FIG. 1).
Then, to completely fill the field-of-view on a planar, normally viewed specular object with uniform light, the BF coaxial illuminators should be larger than and external to the camera lens.
When a specular surface deviates from planar, as in a real beveled edge, for each one degree of full range of surface angle variation, the solid angle of the illumination source must be increased by double in order to appear fully and uniformly illuminated. For example, if a surface has +/-N degree of variance for a total range of 2N degree variance, the total solid angle of light source must be increased by 4N degree, and its diameter from WBFmin to WBF (FIG. 2).
Dark-field illumination uses light impinging upon the object at oblique low angles, and there is detection of the scattered or diffracted rays from the inspected surface, outside the direct specular field-of-view.
This method is used to obtain information primarily from the scratches and particles that have a very small cross-section when inspected under normal angle, but increase their scattered cross-section at low angles. Since the dark-field uses scattered light, this method does not resolve the real size of the defect. This is because at this angle the incident spot size of the laser on the surface can be much larger than the smallest detectable defect.
If the bright-field illuminator has its solid angle of illumination extended to the horizon in all directions, covering practically the dark-field also, the result would be a solid hemisphere. This solid hemisphere includes both the specular and the scattered rays, with very small incident angles, almost parallel to the inspected surface. In the natural world, such continuous uniform illumination geometry is never achieved unless no observer is present. This is because the observer inevitably creates a discontinuity into the field surrounding the object under observation.
In this case, there is a continuous diffuse illumination, inside of a hemisphere. This is a theoretical, ideal, uniform, omnidirectional light source, known in optics as Coblenz sphere or "Total Integrated Sphere" (TIS) illuminator.
U.S. Pat. No. 5,461,417 discloses a practical "Continuous Diffuse Illumination" (CDI) light source, based on this TIS principle.
The basic feature of this prior art U.S. patent is the combination of a diffuse on-axis light source with adjacent off-axis light sources of equal brightness impinging light onto a diffusive hemisphere, to create a continuous diffuse illumination field.
The CDI, described in this prior art U.S. patent, can create a continuous uniform diffuse illuminator if both the on-axis and off-axis light sources are active, at the same time. If only one light source is on, then the, CDI changes itself into in a bright-field illuminator, when only the on-axis source is on. Also the CDI will change itself into a dark-field illuminator, when only the off-axis sources are on. It needs not only an extra mirror-like beamsplitter for separating the bright-field light rays from the reflected light incoming from the inspected sample. But it also needs a minimum of two extra dark-field light sources to be installed inside the diffusive hemisphere. A light switch between these BF and DF sources can only deliver light beams with fixed directions, diameters, and a specific power ratio.